This invention relates to a solid golf ball having satisfactory sensory characteristics (e.g., feel and click) as well as improved durability and flight performance.
For a choice of golf balls, sensory reactions of the ball when hit including an adequately soft feel and an agreeable click are now regarded important. As is known in the art, upon driver or similar shots causing substantial deformation of the ball, the player feels the hardness of an internal region of the ball approximate to its center. A variety of proposals of constructing the golf ball from multiple layers were made for improving the sensory reactions as described, for example, in JP-A 8-336617, 9-56848, 9-299510, 10-328328, and 11-47309. Many of these proposals, however, relate to cores or center spheres having a relatively large diameter of greater than 35 mm and even at minimum, a diameter of roughly 30 mm. They are not necessarily effective for improving feel.
It was also proposed to form a relatively soft center core to a small diameter and enclose the small diameter center core with a hard layer (enclosure or outer core). In this proposal, the outer core is naturally made thick. However, enclosing a soft center core with a hard outer layer substantially results in a core having a large outer diameter. In the region where the ball undergoes substantial deformation, for example, at the time of driver shots, the player feels the hardness of the enclosure or outer core, which feel sometimes becomes hard. If the outer core is too thick, the ball becomes less durable due to the difference in hardness between the center and outer cores.
An object of the invention is to provide a novel and improved golf ball having satisfactory sensory characteristics when hit including a soft feel and an agreeable click as well as improved durability and distance performance.
The invention provides a solid golf ball of multilayer construction of at least four layers, comprising a solid core consisting of a center core and an outer core around the center core, an intermediate layer of at least one layer enclosing the solid core, and a cover enclosing the intermediate layer. The outer core has a higher Shore D hardness than the center core. The solid core has a diameter of up to 27 mm. Then the golf ball has satisfactory sensory reactions upon impact including a soft feel and an agreeable click. Additionally, the ball is fully durable and travels a satisfactory distance.
Preferably, the outer core is formed of a thermoplastic resin base composition. Preferably, the Shore D hardness of the outer core is from 55 to 85 and is at least 5 units higher than the Shore D hardness of the center core. At least one layer of the intermediate layer preferably has a lower Shore D hardness than the outer core. In one preferred embodiment, the intermediate layer closely encloses the surface of the solid core, is formed of a resin base composition, and has a Shore D hardness which is at least 2 units higher than the Shore D hardness of the center core.
The Shore D hardness as used herein is measured according to ASTM D-2240.